1.Field of the invention
The present invention is directed to a method of fabricating an electrode for a discharge lamp which comprises precipitating an aluminum oxide layer in a surface of a Fe-Cr-Al alloy and forming an emitter material on the aluminum oxide layer, and the electrode formed thereby.
2.Description of the prior art
An electrode for a discharge lamp such as a fluorescent lamp is generally fabricated by the following method. That is, a tungsten filament is coated with a carbonate suspension of alkaline earth metals which consist of barium, calcium and strontium, so that a carbonate coated filament is obtained. And then, the carbonate coated filament is heated in vacuum to reduce the carbonates to their oxides. Since the oxides have relatively low work functions, they are able to supply thermo electrons with a low voltage. Therefore, the oxides are utilized as an emitter material for the discharge lamp. By the way, it is known that a lamp life of the discharge lamp depends on a depletion of the emitter material during lamp operation. That is, the emitter material is evaporated from thus fabricated electrode by an ion bombardment of a fill gas which is filled in an envelope of the discharge lamp, and also by heating the electrode at a high temperature of about 1000.degree. C. to operate the lamp. As the thus evaporated emitter material attaches on an inner surface of the envelope, a light output of the lamp is depreciated. Therefore, one way to increase the lamp life is to improve adhesion between the emitter material and a filament. On the other hand, it is desired that a lamp driving circuit for the discharge lamp is reduced in size and weight. In case of using the tungsten filament to the discharge lamp, an electrical conductivity of the tungsten filament remarkably increases as a temperature of the tungsten filament increases. The tungsten filament has a low specific resistance at a room temperature, for example, 5.49.mu..OMEGA.cm at 20.degree. C. The room temperature mentioned in this specification is in a range of 0.degree. C. to 40.degree. C. A large amount of current flows through the tungsten filament as a rush current immediately after a lamp voltage is applied to operate the lamp. However, when the tungsten filament is heated at about 1000.degree. C. during the lamp operation, the specific resistance of the tungsten filament increases to 24.93.mu..OMEGA. cm. As the temperature of the tungsten filament is higher, an amount of current flowing through the tungsten filament decreases. Therefore, the lamp driving circuit corresponding to a large difference of specific resistances of the filament between at the room temperature and the high temperature desired for the discharge lamp. In addition, as the tungsten filament has a low electrical resistance value at the high temperature, as described above, a relatively large amount of current is required to keep the tungsten filament to the high temperature during the lamp operation. Therefore, the lamp driving circuit corresponding to the large amount of current also is needed for the discharge lamp. In order to enable reduction in size and weight of the lamp driving circuit for the discharge lamp, it is desired that a filament material has a small difference of specific resistances of the filament between the room temperature and the high temperature, and also a higher specific resistance value at the high temperature compared with the tungsten filament. The discharge lamp using the filament material can be operated by a small current, which in turn enables the lamp driving circuit to be reduced in size and weight. For example, a heater wire such as a Fe-Cr-Al alloy and Ni-Cr alloy has a small difference of the specific resistances between room temperature and the high temperature, and a relatively high specific resistance value at the high temperature. However, when the heater wire is used as the filament material of the lamp, there is a possibility of the filament deforming or melting during the lamp operation because the heater wire has a lower melting point than tungsten.
Japanese Patent Early Publication [KOKAI] No. 54-121660 describes a cathode for Braun tube. An interface layer which is composed of alumina and barium oxide or silica and barium oxide is formed between the alkaline earth oxides of the emitter material and a cathode substrate. The cathode substrate consists of 20 to 30 wt % of tungsten, 0.01 to 5 wt % of zirconium as a reducer and the balance of nickel. An emission of thermo electrons from the cathode is stably maintained for a long time period by controlling a depletion rate of the reducer, so that a life of the braun tube is increased. The depletion rate of the reducer is controlled by the interface layer.
U.S. Pat. No. 4,836,816 describes about a method of treating a tungsten cathode for increasing a lamp life of the fluorescent lamp. The tungsten cathode is coated with a triple carbonate suspension, and then is placed in a heated oxidizing environment in order to form a diffuse coating consisting of the triple carbonates and tungsten oxide. The triple carbonates consist of barium carbonate, calcium carbonate and strontium carbonate. Subsequently, a cathode activation process is performed to the tungsten cathode with the diffuse coating. That is, the triple carbonates are reduced to their respective oxides by passing heater current through the tungsten cathode. The diffuse coating enhances the chances of forming barium tungstate during the cathode activation process. As a result, the lamp life of the fluorescent lamp is increased and the amount of phosphor darkening is reduced. Although the above-described methods may increase the life of the fluorescent lamp or the braun tube to some degree, it is desirable to have more improved alternative methods.